When we will be working with RFC, sometime our external break points will not work for debugging. The reason behind is when we call any SAP RFC function module from any integration tool, it will be processed on any available application server in the backend, where a user breakpoint can not be triggered to debug the ABAP code in the backend system.
Put a external break point in the RFC code (RFC enable FM or Proxy class implementation).
Go to SRDEBUG tcode and press on Active Debugging.
A pop up screen will appear. Press OK (enter).
Another pop up screen will appear, like the below one.
Let it be as it is and create a different session for your RFC debugging.
Now when you will execute you’re your transaction in ECC SAP R/3 or When the message arrives to SAP R/3 system, a debug session will be started from the point where we have kept the BP.